An embodiment of a light-emitting apparatus of the kind set forth is known from US2005/0269582. That document discloses a semiconductor device comprising a light-emitting layer disposed between an n-type and a p-type region in combination with a ceramic body, which is disposed in a path of the light emitted by the light-emitting layer. The ceramic body is composed of (or includes) a wavelength converting material, such as a phosphor. Typically these ceramic materials are based on Yttrium Aluminum Garnet (YAG), Yttrium Aluminum Silicon Oxo-Nitrides (YSN), Silicon Aluminum Oxo-Nitrides (SiAlON) or Lutetium Aluminum Garnet (LuAG). It is well known in the art that the ‘primary’ light emitted by III-nitride LEDs can be converted into ‘secondary’ light having a longer peak wavelength than the primary light by using the above described ceramic materials. The wavelength converting material can be chosen to obtain a particular peak wavelength of the secondary light. Furthermore, the size and thickness of the ceramic body and/or the concentration of the wavelength converting material can be chosen such that the light emitted by the apparatus is either a mixture of ‘primary’ and ‘secondary’ light or substantially consists of only the ‘secondary’ light. The advantage of this approach lies in the fact that the above-described luminescent ceramic bodies are robust and show a low sensitivity to temperature changes. Furthermore, such luminescent ceramics exhibit (almost) no scattering and therefore have a good conversion efficiency compared to phosphor layers.
As a result of the absence of scatterers the transparent luminescent ceramic body described in US2005/0269582 is a volume emitter with an index of refraction (substantially) larger than 1. This is a considerable drawback, because the primary light transmitted and the secondary light generated by the luminescent ceramic body can be emitted from the body only within the escape cones extending from all body surfaces. In other words, all the light outside these cones is locked inside the ceramic body in so-called waveguide modes. This light is not available for use in the application of the light-emitting apparatus. This unavailability is a clear disadvantage both for étendue critical applications (in which the brightness of the top surface of the ceramic body is of importance) as well as for lumen critical applications (in which the total flux emitted by the ceramic body is of importance). An additional disadvantage of the prior art is that a considerable amount (up to 80% for rectangular bodies) of the light emitted by the transparent luminescent ceramic body is emitted through the side surfaces and therefore not available for use in étendue critical applications.